Rare earth metals and cobalt form various intermetallic compounds. Among them, the intermetallic compounds, RCo.sub.5, whose atomic ratio between rare earth metals (R) and cobalt (Co) is 1:5, exhibit extremely large magnetocrystalline anisotropy and have come to be used as permanent magnet alloy materials. In particular, an energy product of about 24 Mg.Oe, a several times over the conventional alnico or ferrite permanent magnets, is obtainable with a SmCo.sub.5 permanent magnet in which R is samarium (Sm). SmCo.sub.5 is already being commercially produced.
However, the desirable characteristics of a permanent magnet required in recent apparatus such as small rotary machines, small meters, detectors, etc., which utilizes air gap flux caused by a permanent magnet in a magnetic circuit, are a high residual magnetic flux density and a high energy product. Under such circumstances, the Sm.sub.2 Co.sub.17 intermetallic compound attracted attention since it has a higher saturation magnetization, a higher energy product and higher Curie temperature than SmCo.sub.5. In other words, the saturation magnetization of Sm.sub.2 Co.sub.17 reaches 12 KG, whereby, in theory, an energy product of 36 MG.Oe is obtainable. However, coercive force is not obtainable with the Sm.sub.2 Co.sub.17 permanent magnet, and it was necessary to drastically improve the coervice force of the intermetallic compound, Sm.sub.2 Co.sub.17, in order to produce a permanent magnet.
A method to improve the coercive force by partial substitution of Co with Cu or Cu and Fe, etc. has been reported as a measure to improve the coercive force as disclosed in U.S. Pat. No. 3,560,200. Furthermore, it became possible to increase the residual flux density and coercive force and to improve the angularity of demagnetizing characteristic, enabling the achievement of permanent magnet alloys that reach 30 MG.Oe in energy product, (BH).sub.max. However, the composition having such a characteristic was not the stoichiometric composition whose R-Co ratio is 2:17, but it was obtained with an alloy having the intermediate composition between the RCo.sub.5 phase and the R.sub.2 Co.sub.17 phase. As mentioned above, a sufficient magnetic characteristic could not be obtained with R.sub.2 Co.sub.17 whose permanent magnetization has been desirable because of the high magnetocrystalline anistropy saturation magnetization, and Curie temperature. Thus, it has not come to be commercially produced as a permanent magnet material. Furthermore, in order to develop it into a high performance magnet, the composition must truly be of the R.sub.2 Co.sub.17 phase or constitutes mainly of a Co+R.sub.2 Co.sub.17 compound having a greater Co component than in the R.sub.2 Co.sub.17 phase, and it is necessary to partially substitute Co with Fe in order to obtain higher saturation magnetization. The U.S. Pat. No. 4,135,953 reports on permanent magnets produced by adding Cr, Mn, Ti, W, or Mo to R.sub.2 (Co,Fe).sub.17 composition, and alloys of the composition that gained coercive force by sintering the above molded products at 1110.degree.-1180.degree. C., followed by solid solution treatment and heat treatment at 400.degree.-600.degree. C., as well as their manufacturing processes. However, the above invention is a manufacturing process for a liquid phase sintered magnet in which a powder sintering additive with low melting point is mixed in.
The purpose of this invention is to present a manufacturing process for alloys, whose chief constituent is the R(Co, Fe, M).sub.z system (where z is 8.3.about.9.0) stoichiometric composition of R.sub.2 Co.sub.17 phase, by adding a heat treatment process, which is new for rare earth-cobalt magnets, in order to obtain a high coercive force.